The present invention generally concerns a process for preparing aluminum nitride powder. The present invention more particularly concerns preparing aluminum nitride powder that has a surface area greater than 10 square meters per gram (m.sup.2 /g), desirably greater than 15 m.sup.2 /g.
Aluminum nitride synthesis generally occurs via one of four known processes. One well known process involves carbothermally reducing and nitriding alumina (Al.sub.2 O.sub.3 +3C+N.sub.2 .fwdarw.2AlN+3CO). A second well known process directly reacts aluminum metal with nitrogen (2Al+N.sub.2 .fwdarw.2AlN). A less common process reacts aluminum chloride and ammonia in a vapor phase (AlCl.sub.3 +4NH.sub.3 .fwdarw.AlN+3NH.sub.4 Cl). U.S. Pat. No. 3,128,153 discloses an even less common process wherein aluminum phosphide reacts with ammonia (AlP+NH.sub.3 .fwdarw.AlN+1/4P.sub.4 +3/2H.sub.2).
The carbothermal reduction reaction is endothermic and requires approximately 335 kilojoules per gram-mole of aluminum nitride at 1873K. The reaction is generally carried out at a temperature within a range of 1673 to 1973K as disclosed by Kuramoto et al. in U.S. Pat. No. 4,618,592. The resultant aluminum nitride powder is fine enough to allow consolidation to near theoretical density via pressureless sintering in the presence of sintering aids. Higher temperatures generally result in the formation of sintered agglomerates of aluminum nitride particles. The agglomerates are not amenable to densification by pressureless sintering.
The direct reaction of aluminum metal to aluminum nitride is exothermic and generates approximately 328 kilojoules per gram-mole of aluminum nitride at 1800K. Aluminum metal melts at about 933K. The reaction of aluminum and nitrogen starts at about 1073K. The reaction, once initiated, is self-propagating if not controlled. An uncontrolled reaction can reach aluminum nitride sintering temperatures and remain at these temperatures for extended periods of time. The uncontrolled reaction typically yields sintered aluminum nitride agglomerates having a surface area, as determined by Brunauer-Emmett-Teller (BET) analysis, of less than 2 m.sup.2 /g. The agglomerates are not readily amenable to further sintering to densities approaching theoretical density via pressureless sintering techniques. See e.g., F. Skeele et al., "Evaluation of Properties/Performance Relationships for Aluminum Nitride," Ceramic Transactions, Ceramic Powder Science III, 885-893 (1990).
One variation of the direct nitridation process employs plasma reactors to vaporize aluminum metal at temperatures approaching 10,000K and produce ultra-fine aluminum nitride particles. The vaporized metal then reacts with nitrogen, ammonia, or mixtures of nitrogen and ammonia or nitrogen and hydrogen. The resultant aluminum nitride particles have an average particle size of less than 0.1 micrometer and a surface area of approximately 30 m.sup.2 /g. Baba et al., in "Synthesis and Properties of Ultrafine AlN Powder by RF Plasma", Applied Physics Letters, 54 (23), page 2309 (1989), show that these powders can be pressureless sintered to near theoretical density at temperatures as low as 1873K.